The present invention relates to digital switches, in particular digital private branch exchanges (PBX) or local digital exchanges and the testing of a connection between a switch and a subscriber set.
In telephone networks such as for instance a private branch exchange or local digital exchange having telephone sets connected through line boards, there is always a possibility that some line will be incorrect due to a faulty attachment, loosened or oxidized terminal contacts, faults in a line circuit board, etc. Then some suitable method is used for localizing the fault, so that for instance faulty line board can be replaced. There is thus a general need of simple devices for testing the quality of a connection.
Previously, external instruments have been connected for testing line connections by means of bit error measurements, i.e. if a bit sequence transmitted on a line will arrive unchanged to the source transmitting the sequence.
In the published patent application GB-A 2 158 326 a device is disclosed for testing a digital exchange, in particular a connection between the exchange and a subscriber set. A bit sequence formed from pseudo random numbers is transmitted from the exchange on a channel and is transmitted back from a desired point in the connecting path between the exchange and the subscriber set. After each retransmission the bit sequence is tested and its state will indicate if there is a fault. By a successive use of several different retransmitting positions along the connection path the position of a fault can be determined.
In U.S. Pat. No. 4,070,554 a digital telephone system is disclosed comprising a test center for digitally testing the lines between an exchange and the telephone sets. A digital test signal transmitted from the test center is modified in a telephone set at the end of a line under test in a particular way and the modified signal is transmitted back to the exchange. At the test center, the returned signal is compared with a similarly modified non-transmitted test signal. The test center can be located anywhere in the telephone system, allowing any selected subscriber line to be selected for a test. The center is not physically connected to any particular group of subscriber line connections.
A test equipment which also is centrally connected to a digital network is disclosed in the published patent application JP-A 59-109070, publ. No. 60-251751.